In recent years, the rate of use of a skin-stringer structure made of composite material as a shell panel of a fuselage or wings of an airplane has increased. A forming cost of the stringer occupies about 64% of the whole manufacturing cost of the shell panel.
As shown in FIG. 1, a skin-stringer structure 1 or a shell panel made of composite material typically comprises a board-shaped skin 2 which is located on the outer fuselage and painted on an outside surface thereof, and one or more stringers 3 which are attached to an inner surface of the skin 2. For example, the skin 2 is of a plane or curved board made from a plurality of reinforced-fiber sheets stacked one another. Each reinforced-fiber sheet is typically formed such that a large number of reinforced fibers (for example, carbon fibers) are all arranged in one direction (also referred to as “UD” or “Uni-Direction”) to form a sheet shape, and soaked in synthetic resin, such as heat-hardening resin. A desired number of the stacked reinforced-fiber sheets are then stacked to have a desired thickness while they are shaped in a desired shape and then hardened by heat to form an integral plate to be utilized as a part of a shell plate or a skin of an airplane. Similarly, each stringer 3 is formed such that a desired number of the stacked reinforced-fiber sheets are stacked to have a desired thickness and then bent along the direction of the reinforced fibers (typically, along the longitudinal direction of the stacked reinforced-fiber sheets) to form a desired stringer shape. Typically, the stringers 3 are attached to the skin 2 before heating for hardening and, thus, the stringers 3 and the skin 2 are hardened together. Accordingly, each stringer 3 functions as a reinforcement structure for strengthening a skin 2 to resist a stress produced by a twist acting on the skin 2 or a pressure acting on the skin surface.
Typically, many of the skin-stringer structures 1 are made of composite material, such as prepreg, and, generally, as described above, the skin 2 and the stringer 3 are constructed with stacked prepreg sheets.
Conventionally, as disclosed in Japanese Patent Publication No. 2954836, as a typical cross-sectional shape of the stringer 3, a “T-shape” as shown in FIG. 2A or a “J-shape” as shown in FIG. 2B have been adopted (they are shown as inverted shapes in those figures). The T-shape is formed such that belt-shaped stacked prepreg sheets are folded in half to form a web portion 32 of the stringer, and then both open ends are equally bent to opposite directions by 90 degrees to form base-end flange portions 31 of the stringer (lowermost end in the figure) with lower surfaces thereof being attached to the skin 2. On the other hand, the J-shape is formed such that, similarly to the T-shape, belt-shaped stacked prepreg sheets are folded in half to form a web portion 32 of the stringer and then both open ends are equally bent to opposite directions by 90 degrees to form base-end flange portions 31 of the stringer (lowermost end in the figure) with lower surfaces thereof being attached to the skin 2, while a closed or looped end is bent in one direction by 90 degrees to form a tip-end flange portion 33 of the stringer (uppermost end in the figure). Therefore, when applying stacked prepreg sheets of 8 plies, each of the base-end flange portions 31 has a thickness of 8 plies and, the web portion 32 and the tip-end flange portion(s) 33 have a doubled thickness of 16 plies because they are folded in half.
Next, a conventional shaping method of the T-shaped and J-shaped stringers will be explained in detail, referring to FIGS. 3A through 3D and FIGS. 4A through 4D, respectively.
First, in forming the T-shaped stringer, as shown in FIG. 3A, stacked prepreg sheets P of two or more plies before hardening treatment are placed on upper surfaces of a pair of side presses 4. The side presses 4 are press machines extended in the longitudinal direction of the stacked prepreg sheets P (i.e., in the front-and-rear direction which is perpendicular to the drawing sheet) to cover the entire length of the stacked prepreg sheets P, and arranged opposing each other so as to be able to press the stacked prepreg sheets P between mold portions corresponding to a stringer shape, provided at opposing ends of the side presses 4. The side presses 4 are spaced apart by a predetermined gap larger than a thickness of a web portion of the stringer to be yielded. A punch 5 is disposed above the mold portions of the side presses 4 so as to move vertically between the mold portions. The punch 5 also extends in the longitudinal direction of the stacked prepreg sheets P (i.e., in the front-and-rear direction which is perpendicular to the drawing sheet) to cover the entire length of the stacked prepreg sheets P.
As shown in FIG. 3B, while the side presses 4 are maintaining the gap, the punch 5 moves downwardly and then pushes the stacked prepreg sheets P at the center in the width direction thereof (i.e., in the left-and-right direction of the drawing) to fold the stacked prepreg sheets P in half, with the punch 5 being pressed through the gap between the side presses 4. At the same time, a stopper 6, which is provided below the gap so as to move vertically, moves upwardly and then stops at lower surfaces of the side presses 4. Therefore, the stacked prepreg sheets P are stopped at an upper surface of the stopper 6, and are prevented from protruding downwardly from the lower surfaces of the side presses 4.
Then, as shown in FIG. 3C, the punch 5 is moved upwardly and extracted from the stacked prepreg sheets P which are folded in half. For this process, the punch 5 is formed so as to be enlarged at only a tip portion thereof and, thus, the punch 5 is hardly influenced of frictional resistance of the resin in the stacked prepreg sheets P which are sandwiching the punch 5. When the punch 5 is extracted, the side presses 4 press the half-folded stacked prepreg sheets P by a predetermined pressing force to form a web portion of the stringer with a doubled thickness (i.e., a doubled number of plies) of the original stacked prepreg sheets P.
Furthermore, as shown in FIG. 3D, a roller 7 is inserted between open ends of the half-folded stacked prepreg sheets P from above, which protrude upwardly from the upper surfaces of the side presses 4. The roller 7 spreads the open ends of the stacked prepreg sheets P to the left and right, respectively, and presses them between the upper surfaces of the side presses 4 and, therefore, forms a T-shaped stringer.
Next, in forming the J-shaped stringer, as shown in FIG. 4A, stacked prepreg sheets P of two or more plies before hardening treatment are placed on upper surfaces of a pair of side presses 4.
Subsequently, as shown in FIG. 4B, while the side presses 4 are maintaining the gap, the punch 5 moves downwardly and then pushes the stacked prepreg sheets P at the center in the width direction thereof (i.e., in the left-and-right direction of the drawing) to fold the stacked prepreg sheets P in half, as the punch 5 being pressed through the gap between the side presses 4. At the same time, a stopper 6, which is provided below the gap so as to move vertically, moves upwardly and then stops at a position spaced apart from the lower surfaces of the side presses 4 by a predetermined distance. Therefore, the stacked prepreg sheets P are stopped at an upper surface of the stopper 6 with the lower end (closed or looped end) protruding downwardly from the lower surfaces of the side presses 4 at the predetermined distance.
Then, as shown in FIG. 4C, the punch 5 is moved upwardly and extracted from the stacked prepreg sheets P, which are folded in half. When the punch 5 is extracted, the side presses 4 press the half-folded stacked prepreg sheets P by a predetermined pressing force to form a web portion of the stringer with a doubled thickness (i.e., a doubled number of plies) of the original stacked prepreg sheets P.
Furthermore, as shown in FIG. 4D, a roller 7 is inserted between open ends of the half-folded stacked prepreg sheets P from above, which protrude upwardly from the upper surfaces of the side presses 4. The roller 7 spreads the open ends of the stacked prepreg sheets P to the left and right, respectively, and presses them between the upper surfaces of the side presses 4. In addition, another roller 8 traverses from left to right (or right to left) along the lower surfaces of the side presses 4 so that it bends the looped or closed end of the half-folded stacked prepreg sheets P to right while it presses the closed end between the lower surface of the right-side side press 4 and, therefore, forms a J-shaped stringer.
Stringers shaped by such processes described above are arranged with a pre-hardened skin in respective positions. Both the stringers and skin are then heat hardened together and, thus, a skin-stringer structure for a shell panel is produced.
When the prepreg sheets of two or more plies which constitute the stacked prepreg sheets P are bent once in the shaping processes, as shown in FIG. 5A, ends of the plies should slip past each other so as not to be aligned. However, when the stacked prepreg sheets P are further bent to the other direction, ends of the plies should come again in alignment. Such slippage is ideal when bending. However, in conventional stringer shaping method, as shown in FIG. 6A, when the stacked prepreg sheets P are bent once, the slippage does not occur due to a large frictional resistance of the resin in the prepreg sheets. Thus, a phenomenon in which inner ply or plies of the prepreg sheets for bending are distorted (also referred to as “fluctuation” herein) occurs. As shown in FIG. 6B, this “fluctuation” cannot be extended or corrected even if the prepreg sheets are forced to be bent to the other direction in this case because the prepreg sheets are pressurized and restricted between the side presses at a web portion thereof (see FIGS. 4C and 4D). Therefore, the prepreg sheets are hardened by heat with “the fluctuation” resided in the web portion thereof.